1. Field of the Invention
This invention relates to a refractory composition and castable. The refractory composition comprises a refractory aggregate, an ultrafine refractory material having an average particle diameter size less than or equal to about 3 microns, a binder, and a dispersant, wherein all or a portion of the ultrafine refractory material comprises thermal black. This invention also relates to a refractory castable composition comprising the refractory composition of the instant invention and water in an amount sufficient to achieve a castable consistency for casting the refractory castable composition with the use of forms.
Further, this invention relates to an essentially non-slumping, high density, low moisture sprayable refractory castable composition which can be applied without forms, comprising a tempered, pumpable first component comprising the refractory castable composition as disclosed herein, and a second component comprising at least one flocculating agent that is added to the first component at the time of installation of the sprayable refractory castable composition in an amount to prevent slumping. A method of applying the essentially non-slumping, high density, low moisture sprayable refractory castable composition is provided.
2. Brief Description of the Background Art
A considerable amount of work has been done by those skilled in the art to improve the properties of refractory castables. For example, much work has been done to lower the water content required for casting calcium aluminate cement bonded high alumina and fireclay refractory castables. It is well known by those skilled in the art that a reduction in the water content required for casting leads to higher density and lower porosity, ultimately resulting in refractory castables that perform better in service.
The reduction in water content required for casting refractory castables has resulted from the use of dispersants and ultrafine particles in combination with particle packing principles. The dispersants minimize formation of flocs that raise water requirements. More optimum particle packing has been achieved by using progressively finer particles to fill in the voids between the coarser aggregates. Without these ultrafine particles, the voids between the coarser aggregates would fill with water during casting, thus, increasing the amount of water needed in the refractory castable. It will be appreciated by those skilled in the art that a substantial reduction in the amount of water required for casting refractory castables was realized with the discovery that ultrafine particles (i.e. particles having a diameter of less than about three microns) could be included in the products when used with appropriate dispersants. The ultrafine particles used for this purpose have been ultrafine refractory materials, namely, oxides, such as for example, microsilica and finely ground alumina being the most common choices. European Patent EP 0742416 (hereinafter EP '416) discloses a spray operation method for monolithic refractories wherein a fine powder of alumina or fumed silica having a particle size of at most three microns imparts good flowability to the mixed batch of monolithic refractories. U.S. Pat. No. 5,549,745 (Langenohl et al.) (hereinafter U.S. '745) and U.S. Pat. No. 5,512,325 (Langenohl et al.) (hereinafter U.S. '325) disclose a non-slumping, high density, low moisture, low cement sprayable refractory castable composition which can be applied without forms containing a solid flow aid that is microsilica, 1 to 3 micron alumina, or mixtures thereof
Despite their effectiveness in lowering water requirements for casting, there are drawbacks to using microsilica and ultrafine alumina. For example, microsilica typically reduces high temperature refractoriness of refractory castables. In high alumina and fireclay castables bonded with calcium aluminate cement, for example, the microsilica combines with lime from the calcium aluminate cement when the castable is heated, forming low melting point glasses. These glasses cause major disadvantages such as for example, but not limited to, making the castable more prone to creep at high temperatures, decreasing the hot strength and increasing the susceptibility of the castable to chemical corrosion. In an attempt to deal with the problem of glass formation, refractory compositions with either no calcium aluminate cement, or reduced levels of calcium aluminate cement have been developed. The resulting lower lime contents reduce the amount of low melting glass that forms, but refractoriness is still not optimized because the microsilica in the matrix of the castable remain susceptible to chemical alteration and fluxing by constituents commonly found in the environments in which the castables were used. Ultrafine alumina, when used to reduce the water required for casting refractory castables, is a substantial commercial impediment because of its high cost. In addition, ultrafine alumina can have a detrimental effect on the rheology of refractory castables, in particular those that are bonded with calcium aluminate cement. Refractory castables containing calcium aluminate cement and ultrafine alumina can exhibit short working times and poor casting characteristics. While not fully understood, it is believed that the ultrafine alumina provides nucleation sites for precipitation of hydrate phases from solution during mixing and placement of the castables. It is known by those skilled in the art that alumina-lime hydrate phases form on the edges of ultrafine alumina particles in suspensions of ultrafine alumina and calcium aluminate cement in water. It is believed that these hydrate phases affect the morphology of the finest constituents in the refractory castables, thereby adversely affecting rheology and casting characteristics.
Other ultrafine refractory oxides for reducing the amount of water required for casting refractory castables have similar drawbacks. For example, ultrafine chromic oxide is expensive and is undesirable from an environmental standpoint. Ultrafine titania is also expensive and is generally regarded as a flux in refractory systems. Thus, despite the ability of various ultrafine refractory oxides to reduce the water required for casting refractory castables, no ultrafine refractory material heretofore has been found that is wholly satisfactory from either a technical, environmental, or economic standpoint. Further, it will be appreciated by those skilled in the art, that no ultrafine refractory material heretofore has been found that provides for reducing the amount of conventional ultrafine refractory oxides employed or for eliminating the use of conventional ultrafine refractory oxides to fill in the voids between the coarser refractory aggregates while at the same time maintaining particle packing principles for reducing the water content required for casting.
Therefore, in spite of this background material, there remains a very real and substantial need for a refractory composition, castable, and spray mix having an ultrafine refractory material that is capable of lowering the amount of water required for casting or spraying the refractory castable composition while at the same time reducing or eliminating the undesirable characteristics of the ultrafine refractory particles disclosed by the background art.